Step stool



FIG. 1 is a top, front, and right side perspective view of a step stool according to my new design;

FIG. 2 is a top, front, and left side perspective view thereof;

FIG. 3 is a front elevation view thereof;

FIG. 4 is a rear elevation view thereof;

FIG. 5 is a left side elevation view thereof;

FIG. 6 is a right side elevation view thereof;

FIG. 7 is a top plan view thereof; and,

FIG. 8 is a bottom plan view thereof.

The broken lines depict portions of the step stool that form no part of the claimed design. 

CLAIM The ornamental design for a step stool, as shown and described. 